If you own a boat and either occasionally charter your boat, or act as a full-time charter business, you are required by UK law to insure your vessel:
all UK vessels used for commercial gain such as charter yachts up to 24 metres load line length, which go to sea and carry no more than 12 passengers are required by law to comply with the Maritime and Coastguard Agency Small Commercial Vessel and Pilot Boat code and be issued with Certificates by an authorised Certifying authority.

Although charter boat insurance is required by law, the type of policy you obtain is up to you, and this will determine the level of coverage you have. Standard boat insurance policies do sometimes include chart insurance, but the level of cover is not as extensive as that with a specific charter boat insurance policy, and would probably not suffice a full-time company.

The types of coverage available include:
Part time charter coverage which allows the owner to charter the vessel for a specified number of days per annum
Full time charter coverage allows the vessel to be chartered 365 days, and includes day trips, weekly and extended charters
Six-pack charters allows for up to 6 guests on board.
Bareboat charters cover a form of insurance for vessels chartered to a non-owner with no captain or operator provided by the owner.

However, there are additional clauses which could be included into your policy such as:
Hull and Machinery clause covers any damage to the parts of your vessel
Protection and Indemnity clause which allows you to pay out in the event of personal injury to a passenger
Shore excursion which insures your passengers once they have disembarked from your boat
Personal effects will cover any loss of clothing, equipment, cameras and other items.

Choosing an appropriate charter boat insurance policy is important, not only because it is legally required, but because it is able to protect your legal rights, your property and ensures that any damage to your boat, or caused by you to someone elses property is reimbursed.